Communication systems, such as, wireless, optical, spacecraft, satellite, subscriber line, or cellular communication systems, often transmit and receive signals representing encoded and/or compressed data. The coded data or digital information can represent audio, video, voice, programs, software, pictures, graphics, databases, or other material. The data is generally serially provided through a coder to a transmitter which transmits the coded data. The coded data is received by a receiver that includes a decoder for decoding the coded data. The coded data is often provided in a frame or other unit.
Communication systems are discussed below with reference to an exemplary communication system which utilizes a subscriber line as a communication media. For example, a conventional asymmetric digital subscriber line (ADSL) system 10 includes a copper twisted pair analog subscriber line 12, an ADSL modem 14, an ADSL modem 16, a band splitter 18, and a band splitter 20. Line 12 is a plain old telephone service (POTS) local loop or wire connecting a central office 32 of the telephone company to a user's residence 22. In system 10, coded data is communicated across line 12 between modems 14 and 16.
ADSL modem 14 is located in user's residence 22 and provides data to and from subscriber line 12. The data frames or coded data can be provided from line 12 through modem 14 to various equipment (not shown) coupled to modem 14. Equipment, such as, computers, facsimile machines, network devices, servers, or other devices, can be attached to modem 14. Modem 14 communicates with a data network (not shown) via ADSL modem 16 across line 12. Modem 16 receives and transmits signals carrying the data frames from and to line 12 to modem 14. The data network can be coupled to other networks (not shown), including the internet.
At least one analog telephone 26, located in residence 22, can be coupled to subscriber line 12 for communication across line 12 with a telephone switch network 28. Telephone 26 and telephone switch network 28 (e.g., public-switched telephone (PST) network) are conventional systems well-known in the art. Alternatively, other analog equipment, such as, facsimile machines, POTS modems, answering machines, and other telephonic equipment, can be coupled to line 12 in user's residence 22.
System 10 requires that band splitter 18 and band splitter 20 be utilized to separate higher frequency ADSL signals and lower frequency POTS signals. For example, when the user makes a call from residence 22 on telephone 26, lower frequency signals (under 4 kilohertz (kHz)) are provided through band splitter 20 to subscriber line 12 and through band splitter 18 to telephone switch network 28 in central office 32. Band splitter 18 prevents the lower frequency POTS signals from reaching ADSL modem 16. Similarly, band splitter 20 prevents any of the POTS signals from reaching modem 14.
ADSL modem 16 and ADSL modem 14 communicate higher frequency ADSL signals across subscriber line 12. The higher frequency ADSL signals are prevented from reaching telephone 26 and telephone switch network 28 by band splitters 20 and 18, respectively. Splitters 18 and 20 are preferably passive analog filters or other devices which separate lower frequency POTS signals (below 4 kHz) from higher frequency ADSL signals (above 50 kHz).
Communication systems can utilize a variety of coding and decoding schemes to communicate data between locations, such as, residence 22 and office 32. Some schemes arrange the data in packets or frames comprised of multi-bit segments. The frames can be of fixed or variable length and are transmitted and received on a frame-by-frame basis. The frames are often encoded in accordance with an error correction technique.
Error correction techniques often provide error correction data within the frame of data. The error correction data is utilized to ensure that the frame of data has been transmitted and received properly. The error correction data can include parity bits, check symbols, or redundant symbols. For example, Reed-Solomon (R/S) error correction techniques provide N symbols (e.g., bytes) in a frame. Of the N symbols, K symbols are the actual payload or data associated with the frame. The remaining symbols (e.g., N-K) in the frame are utilized to correct symbols within the frame.
In an R/S system, the error processor is able to correct t symbols, where t='(N-K)/2. For example, in a system where N=204 bytes and K=188 bytes, 8 bytes in the frame are correctable, (204-188)/2=8. However, if more than 8 bytes (e.g., more than t symbols) in the frame are in error or corrupted, the frame cannot be corrected by the R/S system, and a frame error has occurred.
According to the R/S technique or other error correction technique, the frame must be retransmitted if there is a frame error (e.g., more then t symbols in error). Retransmitting a frame is disruptive to the communication process and can slow the overall transmission of data considerably. In fact, communication may even be faster at a lower data rate if less frames must be retransmitted due to uncorrectable errors in a frame. Therefore, it is desirous to minimize the number of frames which must be retransmitted in a communication system (e.g., to minimize the number of frame errors).
Frame errors can be the result of a synchronization error between the transmitter and the receiver in the communication system. Synchronization errors can occur due to a loss of signal, to an impulse on the communication medium (e.g., a transmission line), or to a momentary loss of a connection. In communication systems which utilize telephone subscriber lines, synchronization errors can be caused by interference or noise on the subscriber lines. These synchronization errors can often require a retrain operation, which can take a significant amount of time. Also, if synchronization errors go undetected, the communication system can experience a large number of frame errors, which slows the communication process.
Thus, there is a need for a frame error circuit which can detect when a synchronization error has occurred. Further, there is a need for a synchronization error generation circuit which does not require additional data or significant hardware overhead and which can be inexpensively implemented in a frame-coding technique. Further still, there is a need for a digital subscriber line (DSL) modem which can resynchronize frames in response to the determination of a frame synchronization error. Further still, there is a need for a DSL modem which is resynchronized in response to frame errors in a Reed-Solomon frame.